The present invention relates to the field of tape measures and more particularly to locks used to prevent retraction of the retractable tapes used in tape measures.
A tape measure typically includes a case, a metal tape retractably mounted within the case, a retraction means for retracting the tape, a stop for preventing the end of the tape from retracting into the case, a clip to attach the tape measure to clothing, such as a belt, and a lock which releasably secures the tape in an extended position. A common prior art lock includes a toggle switch and a lock shoe attached to the toggle switch, such that when one end of the toggle is depressed, the lock shoe is moved downwardly against the tape which is in turn forced against the inside bottom of the case. Pushing the other end of the toggle releases the lock shoe, allowing the tape to freely retract into the case.
Prior art tape measures employ lock shoes in several configurations. In one such configuration, the lock shoe is a substantially straight finger, which is connected to the toggle by a radial arm. The finger, or lock shoe, is held between the side of the case, which is angled inward, and the side of the tape rolled within the case. When the toggle is depressed, it swings the end of the shoe connected to the radial arm inward with respect to the side of the case, and the opposite end of the shoe is swung outward with respect to the case. Because the ends of the shoe, which is a constant length, swing through an arc, the ends travel laterally with respect to the case bottom. Thus, as the toggle is pushed into the case, the shoe tends to depress the tape against the inner case bottom, eventually wedging the tape with sufficient force to overcome the spring force tending to retract the tape into the case, thereby locking the tape in place.
In U.S. Pat. No. 4,527,334, 7/9/85, Jones, et al., another lock system is disclosed, wherein the tape is attached to a circular housing, and the circular housing is in turn releasably engageable with the lock. In this configuration, the lock has an arm which releasably engages the circular housing on the perimeter of the housing. The lock incorporates a metal coiled spring to assist in biasing the lock against the housing to maintain engagement contact.
U.S. Pat. No. 4,479,617, 10/30/84, Edwards, discloses a lock wherein a reciprocating locking plate has cams which are actuated by the toggle. The toggle has a swing type radial arm which engages the cam, and the toggle motion of engaging and disengaging the toggle causes lateral motion in the locking plate. The locking plate, in turn, pinches the tape against the case bottom to lock the tape in position. A similar lock system is disclosed in U.S. Pat. No. 3,578,259, 10/11/71, Zelnick, et al.
One deficiency of the existing swing lock is that the vertical travel, i.e., the distance the shoe will travel to interferingly lock the tape in place, is limited by the arc through which the shoe end may swing. To accommodate greater arc, and thereby greater vertical travel, the case must be enlarged to permit a longer radial arm. A longer radial arm yields a longer arc, and therefore greater vertical travel. However, tape size is an important cost and utility factor, and thus a larger case increases cost and decreases utility.
Another deficiency of the prior art lock shoes is lock shoe wear. As the tape is pulled past the engaged lock shoe, the end of the lock shoe tends to wear. Likewise, the material of the shoe begins to take on a permanent deformation, i.e., a set, after repeated use. Both of these conditions tend to reduce the effectiveness of the lock shoe over time.
The present invention overcomes these deficiencies of the prior art.